Chemical or “analyte” sensors typically employ a sensing component that is physically or chemically modified in the presence of an analyte. Such sensors include resistance/conductivity sensors, which are generally known as chemiresistors; optical sensors, surface acoustic wave sensors (SAWS), microelectromechanical system (MEMS) sensors, and other types of sensors.
One large class of analyte sensors are the chemiresistor sensors or “chemiresistors.” The general principles behind chemiresistors were demonstrated as early as 1986, when metal ion-phthalocyanine films spread on the surface of interdigitated electrodes showed a change in resistance in response to organic analyte vapor. Charge flowed more easily when atoms in the sensor were brought together and less easily when the atoms were moved apart, which was accomplished by reversibly “swelling” the sensor with analyte or “shrinking” the sensor by removing analyte. Swelling and shrinking alter physical characteristics of the sensor, which produce a detectable change in the electrical conductivity/resistance of the sensor.
More recent analyte sensors contain organic polymers. Analytes typically diffuse into the polymer matrix, thereby changing the conductivity of the polymer by swelling or contracting the matrix and changing the distance between conductive atoms or the pathway taken by the mobile charge. The conductivity of such sensors can be increased through the addition of a “doping” agent into the matrix, such as a conductive salt or carbon black residue (as used in the past) to increase the charge carrying ability of the polymer. Other useful materials include dielectric plasticizers. Organic polymer-based sensors are described in, e.g., Lonergan et al. ((1996) Chem. Mater. 8:2298-2312) and Doleman et al. ((1998) Anal. Chem. 70:4177-90).
In particular examples, a polyaniline polymer doped with carbon black has yielded a class of chemiresistor detectors able to sense amine groups at a sensitivity 1 million-fold greater than that of the human olfactory system (Sotzing et al. (2000) Chem. Mater. 12: 593-595). A polyethylene oxide polymer chemiresistor doped with lithium perchlorate was shown to accurately detect and differentiate between the nerve gas simulants diisopropylmethylphosphonate (DIMP), dimethylformamide (DMMP), and dimethylmethylphosphonate (DMF) (Hughes, et al. (2001) J. Electrochemical Society 148:1-8). Organic polymer-inorganic particle sensors have used in a variety of applications, including those described in U.S. Pat. No. 5,571,401 and the internal references therein.
While conventional sensors often perform adequately for their intended uses, they are not ideally suited to all applications. The repeated expansion and contraction of the sensor matrix due to the influx and efflux of analytes weakens the sensor matrix and leads to reduced performance and eventual failure. It is also difficult to obtain sensors having uniform thin films of a matrix, which leads to variability among sensors. Thick films are generally undesirable because they require an extended period of time to absorb the target analyte, thus increasing the amount of time required for a response to an analyte.
The need exists for more robust and more sensitive analyte sensors. A particularly urgent need exists to rapidly and selectively detect toxic gases, such as hydrogen cyanide, chlorine, carbon monoxide, and hydrogen sulfide, which are volatile chemicals that can rapidly cause death or disability even at low levels of exposure.